1. Field of the Invention
The present invention relates to a variable venturi type carburetor of the type having an intake passage of which cross-sectional area can be varied by negative pressure caused by suction and more particularly to a variable venturi type carburetor which functions as a conventional fixed venturi type carburetor comprising a combination of outer and inner venturis during operation under lower load or the like condition but actuates piston valve during operation under intermediate load, higher load or the like condition to vary the cross-sectional area of the intake passage and moreover can prepare the optimum mixture of fuel over the whole range of engine operation by injecting auxiliary fuel.
2. Description of the Prior Art
It is found that the conventional fixed venturi type carburetor including inner and outer venturis has such a drawback that it is difficult to properly maintain a ratio of air to fuel during operation with the throttle valve kept opened to an intermediate level of opening as well as during operation at a lower rotational speed with the throttle valve fully opened, when the cross-sectional area of the venturi section is determined to generate the maximum output with engine and engine fails to generate its maximum output when the cross-sectional area of the venturi section is intentionally reduced in consideration on the problem relative to the aforementioned ratio of air to fuel, and therefore it cannot obtain satisfactory function over the whole range of operation thereof.
To obviate the foregoing drawback there were already made a variety of proposals for variable venturi type carburetor including a piston valve for varying an effective area of intake passage and a jet needle disposed on the foremost end of the piston valve for controlling both a rate of air passing through the intake passage and a rate of supply of fuel into the latter. One of the hitherto proposed variable venturi type carburetors is intended to atomize fuel and thereby produce well vaporized mixture, as disclosed in Japanese Utility Model Publication No. 4963/1969 (Utility Model Registration No. 884,791) which was filed by the inventor. The carburetor according to his prior invention includes inner and outer venturis and a part of the outer venturi is constituted by the piston valve. The cylindrical inner venturi with a main nozzle incorporated therein is formed integral with the bottom portion of the intake passage and piston valve of which foremost end part is designed in the U-shaped configuration is disposed outside the inner venturi so as to close the outer venturi during operation at a lower speed as required. The jet needle on the foremost end of the piston valve is inserted into the main nozzle through the inner venturi.
However, it is also found that the carburetor according to the inventor's prior invention has a drawback that it is manufactured at an expensive cost because of necessity for machining the jet needle with high accuracy.